Method of synthesizing fatty acid esters

ABSTRACT

A PROCESS OF MANUFACTURING SYNTHETIC FATTY ACID C1-C6 ESTERS FROM STRAIGHT-CHAIN HYDROCARBONS BY CATALYTIC OXIDATION WHEREIN A REACTION CYCLE IS EMPLOYED THAT REDUCES OVER-OXIDATION AND MOLECULAR DEGRADATION SO AS TO PROVIDE HIGHER YIELDS AND REDUCE IMPURITIES. RELATIVELY PURE FATTY ACID ESTERS ARE RECOVERED FROM THE CRUDE OXIDATION PRODUCTS BY A PLURALITY OF ALTERNATIVE PURIFICATION METHODS UTILIZING SAPONIFICATION, HYDROGENATION, POLYOL ESTERIFICATION PROCESSES AS WELL AS EXTRACTION PROCESSES TO REDUCE THE CONTAMINANTS IN THE ULTIMATE PRODUCT SO AS TO YEILD SUBSTANTIALLY PURE PRODUCTS.

United States Patent Oflice 3,816,485 METHOD OF SYNTHESIZING FATTY ACIDESTERS Joseph R. Wechsler, Chicago, Ill., assignor to Stepan ChemicalCompany, Northfield, Ill.

No Drawing. Continuation-impart of application Ser. No.

655,590, July 24, 1967, now abandoned. This application Nov. 13, 1970,Ser. No. 89,491

Int. Cl. 'C07c 51/22, 67/02 US. Cl. 260-410.9 R 16 Claims ABSTRACT OFTHE DISCLOSURE A process of manufacturing synthetic fatty acid C -Cesters from straight-chain hydrocarbons by catalytic oxidation wherein areaction cycle is employed that reduces over-oxidation and moleculardegradation so as to pro vide higher yields and reduce impurities.Relatively pure fatty acid esters are recovered from the crude oxidationproducts by a plurality of alternative purification methods utilizingsaponification, hydrogenation, polyol esterification processes as wellas extraction processes to reduce the contaminants in the ultimateproduct so as to yield substantially pure products.

CROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-partapplication of my copending U.S. Ser. No. 655,590 filed July 24, 1967(now abandoned).

BACKGROUND OF THE INVENTION Field of the invention The present inventionrelates to a process of manufacturing synthetic fatty acid esters. Morespecifically, the invention relates to a process of oxidizingstraight-chain hydrocarbons wherein the majority of the products will beof a carboxylic nature and to a process of purifying the relativelycrude carboxylic oxidation product recovered from such a process toobtain a relatively pure synthetic fatty acid ester.

Description of the prior art Oxidation of aliphatic hydrocarbons to formorganic carboxylic substances is generally one of the oldest organicreactions known. This reaction has not been changed significantly sinceit was first discovered. In principle, the reaction comprises reactingpetroleum wax or some other aliphatic hydrocarbon with an oxygen sourceat high temperatures, in the presence of a catalyst. Generally, thecatalyst utilized is a compound of a polyvalent metal, such as manganeseand/or cobalt and the oxygen source is generally air. The present dayprocess as set forth above suffers from a number of serious drawbacks.The fatty acid materials formed by the known reaction process aresubjected to over-oxidation, which is conductive to the formation of notonly poly-functional compounds but also to substantial moleculardegradation, which prevents the formation of high molecular weight fattyacid materials. In addition, the catalyst used in the known processescreate a number of problems in the efiicient removal and recovery ofsuch catalysts. Moreover, these catalysts tend to form undesirableby-products, such as heavy metal soaps and/or various cationic complexeswhich in turn are extremely difficult to remove from the desired endproduct. Further, the nature of the air input in the known process issuch that a considerably excessive amount of volatile materials areentrained therein and thereby substantially reduce the ultimate yield.Various attempts to capture these volatile products are made, however,they all require the utilization and installation of highly efficientand costly scrubbing equipment.

Patented June 1 1,

Generally, the formed carboxylic compounds obtained from such anoxidation reaction are conventionally removed by treatment with analkali. This removal process is generally performed as a hightemperature saponification ofthe oxidate products so that the obtainedwater solution contains not only carboxylic soaps, but also a largenumber of unreacted or partially reacted hydrocarbons, i.e. parafiins,as well as other various impurities, making subsequent purification verydiflicult and/or uneconomical. I

In accordance the heretofore known procedure, the crude carboxylicoxidation product obtained by saponification of an oxidate of variousparafiin materials contain a substantial amount of impurities, generallyranging from about 20% to about 40%, depending primarily on the methodemployed for its manufacture. These impurities consist of unreactedhydrocarbons, alcohols carbonylic compounds, lactones, variouspolyfunctional compounds and other less identifiable compounds. Variousmeans for removal of these impurities are known. The most common knownmethod is based on solvent extraction of the crude soap solution. Thismethod is disadvantageous in that it necessitates the recovery of thesolvent for economical utilization thereof. Moreover, the solvents usedare always at least partially soluble in the soap solutions so that theyhave to be removed from the product as well as from the extract. Ofcourse, this complicates not only the recovery of the solvents but alsothe purification of the products. Further, this method is only a partialsolution to the purification problem because the effectiveness ofremoving impurities by extraction is a function of distribution betweenthe solvents. Accordingly, an infinite number of extractions arerequired for a complete removal of the impurities. In addition, the useof solvents introduces an element of fire hazard to the overall process.Another known method for the removal of impurities is high pressurestream distillation. While this particular method appears to dispensewith some of the fire hazards involved and the need for recovery of thesolvent, it introduces additional problems. For example, it is necessaryto install extremely costly equipment that is capable of withstandingthe high pressures and temperatures that are required under the highlycorrosive conditions found in such a steam distillation process.Further, this alternative method is again only a partial solution in asmuch as the effectiveness of steam distillation as a function of partialvapor pressures of the impurities so that some of the impurities arealways left behind, especially those having higher boiling points. Inaddition, the various unsaponifiable materials obtained from such aprocess are contaminated by various side reactions during this process,as evidenced by the much darker colors and a tendency to produce ahigher proportion of poly-functional fatty acid materials when suchunsaponifiable materials are re-oxidized. Another severe draw'back fromthis method of purifying the crude products is that a substantial amountof decarboxylation takes place which of course, materially lowers theyield as well as the average molecular weight of the desired product.Further, the drastic conditions presently employed, i.e. temperatureswell in excess of 300 C. and pressures in the order of 200 atmospheresdoes not permanently re move all impurities so that apure product isstill not obtained.

SUMMARY OF THE INVENTION In general, the invention provides a process ofmanufacturing straight-chained mouobasic fatty acid (low molecularweight) esters'i.e., fatty acid ester of low molecular weight alcoholsfrom essentially linear hydrocarbons of an average molecular weightcorresponding to one having 2 to 6 more carbon atoms than the fatty acidprecursors'of the desired range of fatty acid esters and consists ofsubstantially uniformly dispersing an organic peroxide catalyst with thehydrocarbons so as to obtain a mixture thereof and heating the mixtureto a temperature of not more than 160 C. and simultaneously contactingthe mixture with an oxygen containing gas to effect an oxidation of thehydrocarbon so as not to exceed about 40% of free fatty acids by weightcontent of the reaction mixture. Thereafter, an alkali solution is addedand intermixed with the reaction mixture to effect a saponificationthereof. The saponified mixture is collected and a phase separationeffected wherein an organic phase is removed and recycled for additionaloxidation, while the aqueous phase (i.e. the lower phase) is purified toobtain the desired range of fatty acid (low molecular weight) esters. Ina continuous embodiment, when the oxidation of the hydrocarbons attainsan oxidation level not exceeding about 3% of free fatty acids, a portionis continuously withdrawn and purified. In a batch embodiment, when theoxidation of the hydrocarbons attains an oxidation level not exceedingabout 40%, the oxidation is terminated and the batch is purified. Thepurification process includes esterification of the recovered crudeproduct with a polyol, controlled removal of impurities from theresultant material and then transesterification with a selected lowmolecular weight aliphatic alcohol so that the polyol is recovered forreuse and the desired range of fatty acid esters is obtained in arelatively pure state. Alternatively, the crude soap solutions arehydrogenated under temperature-pressure conditions removing a number ofimpurities and a C to C aliphatic alcohol is added to the resultantrelatively concentrated soap solution and then a stoichiometric amountof sulfuric acid is reacted with the resultant solution to form a sodiumsulfate precipitate which is removed from solution. Then a furtheramount of sulfuric acid is added to the remaining solution to effect aphase separation thereof and separating the phases by decanting the toplayer thereof which includes the desired range of the pure fatty acidesters of the C to C alcohol. Alternatively, the relatively concentratedsoap solution is mixed with a solution of sulfuric acid which mayinclude various polar solvent to effect phase separation thereof andmixing the product layer with at least an equivalent amount of a C to Caliphatic alcohol and thereafter reacting the solution with astoichiometric amount of sulfuric acid to precipitate sodium sulfate andremove the same from solution. Thereafter, an additional amount ofsulfuric acid is added for an additional phase separation and the toplayer is decanted from such system and includes essentially pure fattyacid esters of said C to C alcohol.

Accordingly, one of the objects of the present invention is to provide amethod of manufacturing synthetic fatty acid esters from straight-chainhydrocarobns, which meth- -od provides a substantial reduction ofover-oxidation and molecular degradation;

Another object of the present invention is to provide a method ofremoving impurities from crude synthetic fatty "acids obtained in theoxidation of paraffin materials and converting such crude syntheticfatty acids to desired (low molecular weight) esters whereby relativelyeconomical equipment is employed and the chemical tools used in the,removal of the impurities are easily recoverable and the method isrelatively free of fire hazards.

Yet a further object of the invention provides a method of purifyingcrude fatty acid oxidation products obtained from the oxidation ofparaffin materials whereby substantially pure fatty acid (low molecularweight) esters are obtained.

. A further object of the invention is to provide a purithe disclosureof the preferred embodiments thereof in the specification and claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS During the course of theinstant description, reference will be made to an oxidation reaction,and it is to be understood that the scope of this term is intended togenerically imply oxidation reactions utilizing oxygen-containing gas tointroduce at least one end preferably a plurality of oxygen atoms to anorganic hydrocarbon material, such as a parafiin. Accordingly, referenceto an oxidation reaction in the instant disclosure will be understood toapply to any and all reactions which result in the substitution ofoxygen radicals into a molecule of the initial starting materials.

The starting materials or charging stock preferably consists ofsaturated aliphatic straight-chain hydrocarbons containing at leastabout six carbon atoms and up to thirty or forty carbon atoms.Preferably, the starting materials are paraffin hydrocarbons containingfourteen to twenty carbon atoms. The preferred oxygen-containing gas isair, however, other oxygen-containing gases can also be utilized. Inaccordance with the principles of the invention, an oxidizablehydrocarbon starting material is pumped into a suitable reactor chamberwherein it is intimately intermixed with the catalyst. The catalyst maybe mixed with the starting materials during pumping or within thechamber or elsewhere as desired. Preferably, an organic peroxidecatalyst is utilized since since catalyst can be intermixed with thereaction system and allow to remain therein without the necessity offiltering it or otherwise mechanically removing it from the mixture.Preferably, the organic peroxide utilized in the practice of the instantinvention is selected whereby its half-life at the temperature of thereaction is approximately equal to the induction period of the oxidationreaction. Thus, tertiarybutyl hydroperoxide is somewhat slower acting,while certain other alkylol peroxides, such as lauroyl peroxide arerelatively fast. Preferably, organic peroxides having a half-life ofabout 5 to minutes at temperatures in the range of C. to 180 C. arepreferred. Such peroxides include benzoyl peroxide, di-tertiary-butylperoxide, mix ture thereof and other peroxides having similarproperties. The half-life of a catalyst may be defined as the period inwhich the activity of the catalytic substance decreases to approximatelyone half of its initial value. Generally, the ratio of the catalystmaterial to the oxidizable hydrocarbon starting materials, i.e. theparaffin materials, is in the range of about 1:500 to 120,000 andpreferably in the range of 121,000 to 1:2,000.

In accordance with the present invention, raw materials and the catalystare placed in a reactor of convenient size and heated to a temperatureof not more than about C., and preferably in the range of about 120 C.to 160 C., whereupon oxygen-containing gas, such as air, is pumpedthrough the raw materials at a preset rate. After an initial inductionperiod ranging from several minutes to about two hours, dependingprimarily upon the catalyst and the hydrocarbon utilized, the reactionbetween the oxygen and the paraflin materials initiates and maintainsitself. If so desired, a manganese soap soluble in paraffin materialsunder reaction conditions may be utilized to further reduce theinduction time of the reaction. However, the use of this soap mightrequire a filtration step and is thus optional. After the reaction isselfsustaining, the reaction mixture is withdrawn from the reactor at apreset rate as determined by the level of fatty acids therein anddirected through a heat exchanger to a suitable saponification chamberwherein it is treated under efiicient agitation with an aqueous alkalinesolution. The intimate mixture of the oxidation reaction mixture and thealkaline solution is then transferred at a constant rate into aseparator wherein stratification or phase separation occurs. The bottomlayer consists of a soap solution while the top layer consists ofunreacted or partially reacted raw materials which can then betransferred into a second separator or a liquid-liquid centrifuge toremove the last traces of the soap solution. The remainder of thepartially reacted or unreacted oxidation materials is then recycled backto the reaction chamber and again passed through the oxidation reactioncycle just described. A certain amount of fresh raw materials iscontinuously fed into the reactor along with the recovered partiallyreacted mate rials so that the fresh material compensates for theremoved material from the system in the soap solution. The bottom layerof the separator is withdrawn at a relatively constant rate and the soapthus obtained is transformed and purified into the desired range offatty acid (low molecular weight) esters. In the continuous processembodiment, the rate of withdrawal of the reaction mixture from thereactor is so regulated that no more than about 3% of free fatty acidsby weight of the total reactants within the reaction chamber haveaccumulated. This control of withdrawal rate inhibits over-oxidation andthus inhibits formation of multi-functional chemical compound which makepurification difficult. In a typical C14-C13 paraflin oxidationreaction, the fatty acid level is maintained at an average of about 0.1milliequivalent per gram of reaction product and never exceeds about 0.2milliequivalent per gram. However, it will be appreciated that, when thedesired fatty acid esters are of a lower molecular weight, i.e. C C thenin order to maintain a 3% fatty acid level in the withdrawn reactionmixture, the milliequivalent per gram must be increased accordingly.

In accordance with the present invention, the acidic portion of thecrude oxidation reaction product (whether obtained by the abovedescribed continuous reaction or by a batch reaction) may be purified ina number of alternative methods to obtain the desired range of fattyacid esters.

A method of purifying crude oxidation solutions as obtained from theseparator, i.e. crude soap solutions, includes reacting the materialwith an equivalent amount of a mineral acid to separate the crude fattyacids and then reacting these crude acids with a relatively high boilingpolyol so as to form a product having a much higher boiling point rangethan impurities remaining within the re sultant mixture and thereafterremoving such impurities by high temperature vacuum distillation.Thereafter, a relatively low molecular weight aliphatic alcohol is addedto the distillation residue along with an interesterification catalyst,whereby the polyol is released and the fatty acids combined with thealcohol. The polyol is separated and is ready for recycling to theprocess just described and the recovered fatty acid esters of the lowmolecular weight alcohol are relatively pure and ready for use asdesired, or they may be converted into the corresponding range of fattyacids. The method described above can also be used to purify the fattyacid portion of a batch oxidation reaction wherein said portionrepresents 10 to 40% of the total oxidate, by reacting the laterdirectly with a polyol, and then proceeding as explained above, withoutprior saponification.

Another method of purifying the relatively crude fatty acid obtained byacidification of the original soap solution includes selectiveextraction of impurities therefrom with a solution of sulfuric acid in apolar solvent which is reactive with fatty acid carboxylic groups, i.e.C -C alcohols. The extraction process allows impurities to remain 6within one phase of the two-phase system so formed, and substantiallypure fatty acid esters of the C -C alcohol are obtained in the otherphase. By repeating such extraction steps a number of times, a highpurity product can be obtained.

Another method of purifying the crude soap solution obtained from theseparator includes mixing the soap solutions with a predetermined amountof a hydrogenation catalyst, purging the air from around such a mixtureand introducing pressurized hydrogen gas (i.e. at 350 to 650 psi.) andheat to obtain a temperature in the range of about to 300 C. for aperiod of time, and maintaining such conditions while the pressure waswithin the hydrogenation vessel attains a range of about 500 to 2,000psi. After a brief digestion period, i.e. 10 to 60 minutes, the pressureis slowly released and the volatiles are allowed to escape. Thisdistillation is continued until a relatively concentrated solution, i.e.at least 50% strength soap solution is attained. The hydrogenationcatalyst is then removed, as by filtration, and the filtrate is thenfurther processed by dissolving it in an excess amount of a relativelylow molecular weight aliphatic alcohol, i.e. a C to C aliphatic alcohol,and reacting the resultant solution with a stoichiometric amount ofsulfuric acid to form a sodium sulfate precipitate and removing theprecipitate from solution while maintaining the temperature of thesolution at about 10 C. to 50 C. Thereafter, an additional amount ofsulfuric acid is added to the resultant solution to effect a phaseseparation therein and the top layer is decanted. The top layer includesthe desired range of relatively pure fatty acid esters of the lowmolecular weight alcohol.

If desired, the acidic portion of the crude product may be purified in anumber of alternative methods so as to form relatively pure fatty acids.These methods include combined saponification and hydrogenationprocesses as well as extraction processes reducing the level ofcontaminants in the recovered fatty acids. Additional concepts of thisinvention are disclosed and claimed in my copending U.S. Ser. No.89,490, filed Nov. 13, 1970, now Pat. No. 3,708,513.

Yet another method of purifying crude C C esters obtained from a sourcethereof, such as initially available from one of the precedingembodiments yielding crude C -C esters, comprises mixing the crude C -Cesters with a suitable solvent for any impurities therein and effectinga phase separation. A preferred solvent of this nature is relativelystrong (60% to 94%) sulfuric acid. A relatively small amount of sulfuricacid is mixed with a portion of crude C -C esters so as to form atwo-layer system, which is then separated into its individual layers.The lower layer contains a majority of any impurities and the upperlayer contains a majority of the C -C esters and an amount of impuritiessubstantially less than that originally present. This procedure may berepeated any number of times to obtain a desired degree of purity in thefatty esters.

As indicated previously, the preferred raw materials are straight-chainnormal paraffin compounds, particularly those having from 10 to 24carbon atoms. Expressed in another way, particularly since the parafiinmaterials are generally not obtainable in any one particular pure statebut generally consist of a mixture of various molecular weightcompounds, the starting parafiin materials should have a moleculardistribution such that the molecular average value substantiallycorresponds to the molecular weight of the fatty acid precursors of thedesired range of fatty acid (low molecular Weight) esters plus 2 to 6additional carbon atoms, i.e., the oxidation process of the instantinvention generally removes 2 to 6 carbon atoms in forming the fattyacid precursors which are then esteritied with a desired low molecularweight alcohol to yield the desired fatty acid low molecular Weightesters.

When the paraffinic materials are intimately dispersed with air atelevated temperature, there is an induction period before the reactionactually begins, which varies with the particular conditions employed.The preferred catalysts of the invention reduce normal induction periodof to 15 hours to an induction period of less than 2 hours, dependingupon the particular catalyst employed. Once the induction period hasoccurred, the oxidation reaction becomes self-sustaining and the crudereaction mixture products can be steadily and continuously removed fromthe reaction chamber at a preset rate of withdrawal, as describedearlier. The steady and continuous removal of reaction mixture, whichincludes carboxylic compounds formed during the oxidation reaction, fromthe reaction zone or chamber inhibits re-oxidation of such compoundsince this would result in diand other poly-functional compounds whichare undesirable in that their presence renders purification extremelydifiicult. Further, such steady and continuous removal of the variouscarboxylic compounds formed, produces an additional beneficial effect inas much as the average molecular weight of the product obtainedtherefrom is significantly higher than the products obtained from adiscontinuous or batch-type process. It is therefore apparent that thesteady and continuous removal of the carboxylic compounds from thereaction zones significantly reduces molecular degradation. This isespecially surprising since workers in the art have generally suggestedthat oxygen attack on a parafiin molecule is directed so that theend-product would be of a molecular size essentially half of thestarting material.

A further beneficial effect which is immediately measurable is that theamount of esters formed in the oxidation products during any given timeinterval is substantially smaller than the amount of free acids formedduring the same interval. This is quite surprising since in a batchtypeprocess the amount of esters formed is usually found to be equal orgreater than the amount of free acids formed. During a typical oxidationreaction of the invention, the ratio between esters and fatty acidswithin the reaction chamber is practically constant throughout thereaction period, although acids are being continuously removed from thereaction mixture while the esters remain therein.

A general characteristic of this type of oxidation is the formation ofwater of reaction. Generally, this water of reaction is collected in anappropriate scrubber system. The water layer generally contains anappreciable amount of volatiles and water soluble products, such asformic, acidic, and propionic acids, as Well as apreciable amounts ofperacids and hydrogen peroxide, which latter compounds are especiallyuseful in epoxidation applications.

In order to achieve a process allowing the continuous removal ofreaction products and recycling of raw materials (i.e. the unreacted andpartially reacted starting materials) so as to allow such process to runfor an infinite length of time, operational conditions have to becarefully adjusted so that the chemical composition of the materials inthe reaction zone will be essentially the same at any given time duringthe process. This is achieved by proper adjustment of several variables,which include: temperature of the reaction; residence time;concentration and rate of alkaline solution and temperature of thealkali treatment.

The temperature of the reaction is carefully selected and controlled.The rate of reaction increases with temperature but so does re-oxidationand molecular degradation. However, if the temperature is too low thereaction will slow down and finally stop. The preferred temperature ofreaction in the present invention is in the range of 120 C. to 160 C.whereby a balance between the speed of reaction and avoidance ofreoxidation and degradation is achieved.

The residence time of the various chemical constituents in the reactionszone, is of course, somewhat dependent upon temperature of the reactionsince the temperature regulates the reaction rate. Generally, theshorter the residence time, the less undesirable side reactions and thecleaner the product will be. However, too short a residence time doesnot yield sufficient product, even though such product might beexceptionally pure. It is therefore preferred to have a residence timein the range of about 20 to minutes.

The concentration of the alkaline solution must be carefully selected inorder to obtain the best results. If an alkaline solution is utilizedwhich it too dilute, the soap solution tends to form emulsions whichtake a long time for Stratification or phase separation and therebyupsets the timing of the cycle. On the other hand, if the alkalinesolution utilized is too concentrated, then the soap solution willseparate out faster but will have unfavorable solubility properties.Generally, it is preferred to utilize an alkali, such as sodiumhydroxide, with a concentration range of about 2% to 15% by weight inwater to obtain the best results. The rate of flow of the alkalinesolution into the reaction mixture is adjusted in such a manner thatthere is preferably a slight excess of alkali over the total carboxyliccontent in the oxidation product at all times. In other words, the pH ofthe overall mixture is maintained substantially in the range of 8.0 to12. If this pH is not maintained, extraction of the fatty acids becomeserratic, apparently due to the tendency of soap to form complexes withfree fatty acid at a more acidic pH. As will be appreciated, any alkalimay be utilized, although sodium hydroxide is preferred from asolubility and economic consideration.

The temperature of the alkalioxidation product mixture is also carefullycontrolled. If this temperature is allowed to become excessively high,excess material will be extracted during each cycle and will contaminatethe product. This will also tend to slow down the reaction andeventually stop it, since essential reaction intermediates, chiefiyperoxides are removed or destroyed and cannot be regenerated fast enoughto maintain the required rate of reaction. On the other hand, if thesaponification temperatures are allowed to become too low, there is atendency to form emulsions which separate too slowly to allow acontinuous operation. Preferably, temperature is regulated so as to bein the range of about 30 C. to 60 C. and thereby provide effective andselective removal of the desired range of fatty acid precursors as wellas a reasonably fast phase separation without a significant reduction inperoxide content of the reaction intermediates.

The rate of oxygen-containing gas pumped through the reactor influencesthe reaction to a limited extent over a wide range of rates.Essentially, the rate of oxygen containing gas, such as air, must behigh enough to insure an excess of oxygen over and above the amount ofoxygen required to form the expected amount of oxidation products perunit time. Generally, a sufiicient excess is obtained by the use of aflow rate of air corresponding to about 250 to 1,000 ml. of air per kg.of hydrocarbon per minute, or about 4 to 16 times the theoretical oxygenrequirement of the paraffin material to oxidize it to the desired fattyacids. Accordingly, the ratio of oxygen-containing gas to the parafiinmaterials is generally in the range of about 2:1 to 20:1. By keeping theflow of air on the lower side of the above range, excessive losses ofvolatile reactants are avoided and overall yields are substantiallyincreased without affecting the chemistry of the reaction to anyappreciable extent. Moreover, by keeping the air flow at a minimumwithin the aforesaid range, the rate of ester formation is substantiallydepressed both in a continuous-type reaction and also in a batchtypereaction. One explanation for this occurrence is that at higher rates ofair flow, water of esterification is more quickly removed therebyallowing esterification to proceed at a faster rate and under thepresent conditions, where a minimum air flow is utilized, the water ofesterification tends to accumulate and thereby retard esterification.Preferably, the air or other oxygen-containing gas is intimatelydispersed through the reaction mixture by means of any convenientmechanical means, such as increased agitation of the overall mixtureand/or utilization of a fine spray contacting the oxygen containing gaswith the hydrocarbon materials. Generally, a sparge system wherein theair enters into a reactor through fast moving agitator blades havingopenings therein yields satisfactory results. Further, as the reactionproceeds and the oxidation products are formed, the degree of dispersionof air also increases thus making it unnecessary to use anysurfactant-based surface tension depressors or other dispersion agentsto effect the intimate dispersion between oxygen and the hydrocarbons.

The reaction is essentially exothermic and the reaction zone must beprovided with an eflicient heat exchanger in order to maintain arelatively constant reaction temperature. The heat exchanger is of suchsize and capacity as to be capable of maintaining the conditions inaccordance with the volume of the reaction mixture.

The phase separation of the saponified oxidation products do not allowfor ideal efiiciency, and trace amounts of soap, water and free alkaliare occasionally entrained by the recycle materials into the reactionzone. Generally, such small amounts of entrained contaminants do notinfluence the reaction to any appreciable extent except for theformation of minor solid particles which may tend to collect in spotsthroughout the reaction apparatus and threaten to clog the flow of thereaction mixture. However, convenient means, such as traps or the likemay be placed in the conduits leading from the separator system to thereaction chamber to remove such solid particles.

A preferred means of utilizing a continuous process operation with aminimum of supervision includes the use of automatic leveling devices tocontrol the flow of fluids automatically as a function of the presentlevels and interfaces whereby once the equilibrium conditions have beenattained, such devices continue to maintain the equilibrium conditions.By way of illustrating the preferred oxidation process of the presentinvention, examples 1 and 2 are set forth hereinafter so that theprinciples of the invention may be better understood.

As indicated earlier, the crude oxidation products obtained from theoxidation reaction of paraflin materials described hereinabove may bepurified in a number of improved and novel methods. One particularmethod of purifying such crude synthetic fatty acid products obtained bythe oxidation of hydrocarbons involves the reaction of the products witha relatively high boiling material, such as a polyol in a manner so asto produce a mixture containing a higher boiling point polyestermaterial and lower boiling impurities. The impurities are then removedby high temperature vacuum distillation and thereaftertransesterification with a desired low molecular weight aliphaticalcohol, in the presence of an interesterification catalyst isperformed, and the polyol is recovered in a relatively pure state readyfor reuse Without requirement for repurification or any additionalmanipulation. The high boiling reactant materials are preferablypolyols, however, polyamines are also utilizable and the preferredpolyols are selected from the group consisting essentially ofpentaerythritol, glycerol, glycols and other similarly characterizedpolyols. In accordance with the principles of the invention, therelatively crude soap solutions which obtained by saponification of theoxidation products of the paraffins are subjected to flash distillationconditions so as to remove the desired fatty acid precursors from higherboiling polyfunctional impurity materials which tend to interfere withthe purification operation. During such flash-distillation procedure, acertain amount of resinification occurs, so that the residue appears tobe quite large. A great majority of the resinification is apparently dueto esterification, so that by submitting the residue to an efiicientsaponification process, about half of the material can be recovered asrelatively clean fatty acid precursor material.

The distillation is carried out in such a manner that mostmonofunctional materials will be removed with the distillate. Forexample, where the crude fatty acids consist primarily of C -C acidproducts, the material is distilled until a vapor temperature of about190 C. is reached and a reduced pressure or vacuum is used generally inthe range of about 2 to 10 mm. Hg. The selected final temperaturedepends, of course, on the molecular size and distribution of theproduct being purified. Generally, it will be found that thedistillation reaches an end point which is close to the point wherelittle, if any, monofunctional materials remain in the residue.

This initial distillation step may be omitted and the crude productsubmitted directly to the procedure outlined hereinafter yielding asatisfactory product. However, it will be found that if this procedureis utilized, the recovery of the polyol is greatly reduced. As indicatedearlier, the crude oxidation obtained from the oxidation reaction ofparaflin materials described herein may be purified in a number ofimproved and novel methods.

One particular method of purifying such crude synthetic fatty acidprecursors to obtain fatty acid (low molecular weight) esters comprisescollecting the distillate from the flash distillation procedure, or ifdesired, omitting the flash distillation and merely utilizing the crudefatty acid products and reacting them with a high boiling reactiveselected from the group consisting essentially of relatively highboiling polyols and preferably is pentaerythritol under conditionsconductive for complete esterification of fatty acid precursors withinsuch starting materials. Generally, a slight excess of the high reactivematerial, i.e., the polyol is preferred to insure complete reaction ofall of the fatty acids in the distillate-or starting materials. Theesterification reaction is preferably carried out under inert blanket,such as nitrogen, to avoid side reactions which tend to occur in thepresence of air. An esterification catalyst may be utilized, if desired,however, this is optional. The reaction mixture is heated andcontinuously agitated to insure intimate contact between the variouscomponents thereof. As the reaction progresses, water of esterificationis formed and constantly removed. Preferably, an azeotropic agent, suchas xylene, is added to the reaction mixture to remove the water ofesterification. The temperature is maintained substantially below about300 C. for a period of time until no further water of esterification isbeing collected. The preferred end point of the esterification reactionis ascertained by the free acid content in the reaction mixture.Preferably, free acid content in the mixture should generally be in therange of about 0.005 to 0.5 milliequivalents per gram of reactionmixture. It will be appreciated, of course, that the esterificationreaction could proceed until free fatty acid content is brought down tosubstantially zero, however, practical considerations dictate otherwise.

When the esterification is substantially complete, the azeotropic agentis stripped off, as by distillation, and the reaction mixture is allowedto cool to about 50 C. to C. under an inert atmosphere and a relativelyhigh vacuum, generally in the range of 2 to 10 mm. Hg is applied andimpurity, i.e. non-acids, are distilled off for a period of time untilthe vapor temperature reaches a point well above the temperatureselected for the final fraction of the ultimate product under the vacuumconditions. For example, if the final product is to be fractionated atC., then the distillate temperature is maintained at about C. to C. Thisinsures that practically no impurities will co-distill with the fractionof pure products obtained subsequently.

The stripping process can, of course, be extended to a point where thedistillation slows down due to a lack of mono-functional impurities, butthis entails the risk of losing some of the polyol, because the lowestfraction of the polyesters have a tendency to overlap with the highestboiling impurity. 1

The distillate obtained in the last step (hereafter called non-acids)can be re-cycled as oxidation raw material in the oxidation processdescribed hereinbefore. Such nonacids contain mainly partially reactedand/or unreacted paraflin and chemical intermediates.

The stripped polyesters are allowed to cool down to almost ambienttemperature and then dissolve in a molar excess of at least fiveequivalents of a low boiling aliphatic alcohol, ie a C -C aliphaticalcohol, such as methanol, and a small amount of an efficientinteresterification catalyst, such as sodium methylate (NaOCH Thepolyol, i.e. pentaerythritol, separates out and esters of the lowboiling alcohol are formed with the range of fatty acid precursorspresent. Generally, a time period in the range of 15 to 60 minutes issufficient to attain fairly complete separation. The excess amount oflow boiling alcohol is then stripped off and the separated polyolrecovered for recycling in a process described. The resulting fatty acidesters can be fractionated to obtain better separation thereof.Additionally, such fatty acid esters may be readily converted into thefatty acids if desired.

The fractions of fatty acid esters obtained by the practice of theinvention are of high purity and are substantially in the range of 9899%pure which generally compares with the order of magnitude of 94-96%purity obtained by high pressure steam treatment or 89-92% as obtainedby present day solvent extractions.

The residue from the final fractionation can be combined with theresidue obtained from the initial distillation and resaponified so thatabout half of the combined residues can be recovered as fatty acidprecursors. Of course, such precursors, i.e. the fatty acids, can beutilized as such. It has been found that the acids recovered from suchresidue have a surprisingly high degree of purity, which may beexplained by the fact that most of the distillated impurities havealready been removed.

Another method of purifying crude synthetic fatty acid products obtainedby the oxidation of hydrocarbons involves the combination ofhydrogenation and saponification process as well as extractionprocesses.

In accordance with this method, a crude soap solution of about 10% to40% and preferably 25% strength is fed into an appropriate pressurevessel capable of handling corrosive materials at elevated pressure andtemperature conditions. A crude-soap solution of a given concentrationor strength is mixed with a predetermined amount of a hydrogenationcatalyst and air is purged from the pressure vessel. Hydrogen gas isintroduced into the system under pressure in the range of about 350 to650 p.s.i. while heat is simultaneously added so that the mixtureattains a temperature in the range of about 190 C. to 300 C. and theseconditions are maintained for a digestive period of time while thepressure within the system increases up to about 2,000 p.s.i. After adigestion period of about 10 to 60 minutes, the pressure is graduallyreleased through an opening at the top of the vessel allowing steam andother volatiles to escape into a suitable condenser system. The steamentrains various gross impurities which can be condensed to form an oillayer that is easily recoverable from the condensate by decantation. Theoil layer is of a light color and consists primarily of unreactedparafiins, alcohols, ketones and other less identifiable chemicals. Itis significant that practically no esters can be found in this oil.

The steam distillation is continued until the crude soap solutionreaches a relatively concentrated stage, i.e. at least 50% strength, asindicated by the amount of the condensate. The soap solution is thendischarged, and the catalyst is removed, as by filtration. The filtrateis then further processed to obtain the desired range of relatively purefatty acid low molecular weight esters, or, if desired the pure fattyacids.

The above described hydrogenation process can be carried out in such away that the charging of the crude soap solution and the steamdistillation therefrom proceeds in a substantially continuous manner.For instance, the crude soap solution coming from a continuous oxidationprocess is pumped into a pressure vessel where it is mixed with acatalyst and hydrogen and is then pressurized and heated to a highertemperature while steam and concentrated soap are expelled at acontrolled rate. All these various flows are adjusted in such a mannerthat a preset residence time is achieved and a steady level of liquidsis maintained throughout the system.

The relatively concentrated soap solutions can be treated a number ofdifferent ways to obtain the desired fatty acid low molecular weightesters. As indicated, the relatively concentrated soap solutions (i.e.ones having about 50% to about strength) are cooled to a temperaturebelow about 70 C. and the hydrogenation catalyst is removed. Thereafter,at least an equivalent and preferably 1 to 3 volume equivalents of a Cto C aliphatic alcohol is added to this concentrated soap solution andwell agitated to obtain a substantially uniform solution. Thereafter, astoichiometric amount of sulfuric acid is added to this solution andallowed to react therewith so as to form a sodium sulfate salt whichimmediately precipitates. The precipitated sodium salt is removed fromthe solution, as by simple filtration, and washed with a suitablesolvent, such as methanol, and recovered in a relatively pure state. Theremaining solution is then provided with a relatively small amount ofadditional sulfuric acid (having a concentration in the range of about60% to 94%) whereupon a clear cut phase separation occurs, the top layercontaining substantially all of the desired range of pure fatty acidesters of the C to C alcohol and the bottom layer containingsubstantially all of the mineral acids and various other impurities. Ithas been found that by repeated extraction, with various mixtures ofacid and alcohol the amounts of impurities remaining in the fatty acidmethyl esters are drastically reduced. This appears to be primarily afunction of the selective solubility displayed by the acid/ alcoholsolvent system, which shows a marked aflinity for poly-functionalchemicals, such as lactones.

The very rapid and high yield esterification process under a mildreaction condition is made possible by the efficient removal of water ofreaction, partially by the sodium sulfate salt that is formed andpartially by the sulfuric acid itself. It has been found that thisprinciple is applicable to a wide variety of alcohols and fatty acids,limited only by solubility considerations. Thus, mixtures of sulfuricacid with any relatively low boiling aliphatic alcohol, althoughmethanol is preferred, satisfactorily participates in the esterificationreaction just described and removes impurities therefrom so thatrelatively pure fatty acid esters are obtainable. For instance,isopropanol, tertiary-butanol, ethanol, etc. are effective components inthe acid-alcohol solvent mixture for the purposes as explainedhereinbefore. Preferably methanol is utilized as it is more abundant andthe fatty acid methyl esters find great utility. When it is desired topurify the methyl ester, (or other low molecular weight ester) obtainedby the steps described above to a higher degree, additional mixing andseparating from a mixture of sulfuric acid and methanol at a ratio ofsulfuric acid to such alcohol being substantially in the range of 1:10to 10:1 will greatly reduce the contaminants present.

It has also been found that certain polar solvents other than alcohols,which are substantially non-reactive with fatty acid carboxylic groupsalso perform satisfactorily as impurity removing materials, when mixedwith sulfuric acid. Examples of such additional polar solvents includeacetone, dioxane, methyl butyl ketone, etc. The ratio of sulfuric acidto polar solvents likewise range from about 1:10 to 10:1. As indicated,repeated extractions are utilized to further increase the purity of theproduct, particularly by the use of a counter current flow process.

The temperature of the extraction solution has been found to besignificant in that if it is too high, scorching oxidized.

. 'Thefatty acid methyl esters obtained by the above- .described methodsare then submitted to fractionation, in' order to obtain products havingthe desired range of molecular weight. These products are found to bewaterwhite, with the exception of the highest fraction which has aslight pale hue. The color stability is relatively good. However, ifsuch products are to be used as raw materials the manufacture of yetother products wherein absolute absence of coloring material is desired,additional treatments are generally necessary for the removal of tracesof chromophores from such ultimate products. The fractionated productsmay be treated with sulfuric acid of about 60-70% concentration andpreferably less than 94% concentration, under mild conditions, followedby a treatment with a mild alkali solution, such as a bicarbonate, ordiluted sodium hydroxide, to increase the stability of the color. Thesequence of acid-alkali or alkaliacid treatment is important and dependson the medium wherein a further reaction of the product will occur.

Thus, for instance, when a reaction is intended which requires an alkalicatalyst, it is preferable to follow the acid-alkali sequence,. while anacid catalyst would impose a reverse order.

' When it is desired to run the oxidation process hereinbefore describedin a discontinuous or batch process, the recovery of the fatty acid lowmolecular weight esters from the oxidate can be greatly enhanced byapplying the principles of the present invention. For example, after theoxidation of a batch has reached the desired free acid content (i.e.about to 40% by weight), the whole oxidate can be reacted with acalculated amount of a polyol, such as pentaerythritol, untilpractically complete esterification is achieved. All of the unoxidizedand partially oxidized non-acids can then be removed by vacuumdistillation and the obtained distillate recycled to the next oxidationbatch. The remaining polyol esters can then be transformed into methylesters by the hereinbefore described interesterification reaction. Ithas been found that the recovery of the polyol is not as efficient inthe batch process, apparently due to the presence of polyfunctionalcompounds, but the additional loss is more than compensated for by thefact that this process requires neither alkaline or mineral acid and thenumber of steps involved is greatly reduced.

'A number of examples will now be set forth to further illustrate butnot limit the principles of the invention.

V EXAMPLE I 6,335 grams of raw materials consisting essentially of amixture of normaltetradecane, pentadecane, and hexa- '1 One gram ofdi-tertiary butyl peroxide was placed in the -reactor and the materialheated to 140 C. Air was sparged through the reactor at a rate of about8 liters per minute (corresponding to 3.31 kg. oil/min.) and thereaction set in almost at once, as evidenced by formation of water ofreaction which was collected in a water trap.

When about 4 ml. of water' was collected, the raw materials were allowedto circulate in the system and the continuous cycle started at a rateadjusted to one hour residence time, which corresponds to a maximumfatty acid content of about 3% by weight of total reactants within thereactor. The adjustment was by means of adjustable Teflon stop cockscontrolling all gravity flows and with conventional pumps. A NaOHsolution was charged into the alkali supply vessel at a 2.5% strengthand its rate of flow was adjusted to provide enough alkali to neutralizethe free fatty acids in the reactants at the saponification chamber.This cycle was maintained around the clock for 84 hours and at the endof this time period the reaction was stopped. During the reaction time,fresh materials (but no additional catalyst, nor any preoxidizedmaterial) was continuously fed to the system at a rate which wouldcompensate for the material removed from the system by the alkalisolution and by periodic sampling for analysis. The total charge of rawmaterials during the cycle was 14,820 grams. At the end of the run,8,044 grams of oil were accounted for thus:

Grams In the apparatus 6,390 Removed for sampling 962 Recovered from thescrubber system 692 Accordingly, the consumed materials amounted to6,776 grams. The soap solution collected in the soap reservoir wasacidified with H 50 to a pH below 3.5, and the obtained layer of crudereaction products, i.e. unpurified fatty acids, weighed 5694 grams forabout a 84 yield. As will be appreciated, the acidification wasperformed at this time merely to ascertain the results of the oxidationprocess and under natural operating conditions it is more practical tocontinue directly with one of the purification steps discussed earlier.

Of this crude oxidation product, 5,100 grams was resaponified and thegross impurities removed by repeated extraction with petroleum ether,whereupon the soap solution was acidified and 3,380 grams of fatty acidand 1,465 grams of oil were recovered. This oil was suitable for use asa recycle material into the oxidation process. The fatty acids thusobtained had an average molecular weight of 186, generally correspondingto undecanoic acid and had a molecular distribution as follows:

Faty acids: Percent C -C 6.5 C -C 26.8 G -C 54.9 15- 1e 1 L8 from, whichwas found to contain the desired range of relatively pure fatty acidmethyl esters.

EXAMPLE II 9,100 grams of a starting material consisting essentially ofa mixture of normal tetradecane (29.4% Pentadecane (42.9% and hexadecane(23.6%) was charged into the apparatus system described in conjunctionwith Example I by placing 2,400 grams into the reactor and dividing therest of the material between the two separators, the saponificationvessel and the reservoir vessel. In order to avoid an induction period,the starting material was preoxidized to an acid content of 0.15me./'g., and the free acid removed by extraction with an alkalinesolution. One

gram of di-tertiary butyl peroxide was placed in the reactor and thematerial heated to 140 C. Air was sparged through the reactor at a rateof 2.5 liters per minute, or 1 liter air/kg oil/minute, and the reactionset in almost at once, as evidenced by water of reaction starting tocollect in a trap. When about 4 ml. of water was collected, thecontinuous cycle was started at a rate of one hour residence time in thereactor (i.e. corresponding to approximately a 3% fatty acid formation).A NaOH solution was charged into the alkali supply vessel at a 9.3%strength and its rate of flow into the saponification vessel wasadjusted to provide an amount of alkali corresponding to about excessover the amount calculated to neutralize the total carboxylic content inthe reactant at the saponification chamber. In order to help break upthe emulsion, the temperature in the saponification vessel wasmaintained at about 40 to 45 C. No other electrolyte was used in theNaOH solution, and the rate of separation of emulsion was satisfactory.This cycle was maintained around the clock for 108 hours, at the end ofwhich time period it was stopped. The general equilibrium conditions ofthe reaction were noted and the ratio of free fatty acid to estersremained essentially constant. During this time fresh material wascontinuously fed to the system at a rate adjusted to compensate for thematerials removed from the system during the reaction period. In orderto simulate ultimate batch-type conditions, the fresh material wasidentical with the material used for the initial charge. No additionalcatalyst was used. These fresh materials brought the total charge of rawmaterial to 22,640 grams.

At the end of the run, 11,015 grams of oil was accounted for thus:

Accordingly, the consumed materials amounted to 11,625 grams. The soapsolution was collected in the soap reservoir vessel and acidified with H50 to a pH 3 (again to merely ascertain the results of the oxidationprocess) and a crude oxidation reaction mixture so obtained weighed11,480 grams for a yield of 98.8%.

500 grams of this crude reaction mixture was dissolved in a solution of100 grams NaOH in 1500 cc. of H 0 to recreate pre-acidificationconditions. The so-obtained soap solution was then charged into anautoclave of 4 liter capacity and heated to about 250 C. under pressure.Gross impurities were removed by steam distillation and the concentratedsoap solution was cooled and acidified. In this manner 75 grams ofrecycled oil was collected and 351 grams of synthetic fatty acids wererecovered. The recycle oil had a carbonyl content of about 0.68 me./g.and an OH content of 2.54 me./g. The synthetic fatty acids recoveredwere analyzed to have a number average molecular weight of 172,generally corresponding to decanoic acid, and had a moleculardistribution as follows:

as determined by gas chromatography. These fatty acids were then reactedwith a C to C aliphatic alcohol and purified to obtain the desired rangeof fatty acid low molecular weight esters.

EXAMPLE III An oxidation reaction was carried out in accordance with theprocedure described in Examples I and II. After the oxidate wassaponified and the soap solution acidified,

the crude fatty acids were found to have a mean molecular weight of 175,a free acid content of 73.8% and an ester content of 0.42 me./g.

1937 g. of this crude fatty acid reaction product was distilled under2.5 mm. Hg vacuum until the vapor temperature reached 188 C. 1525 g.distillate was collected and analyzed to contain 0.67 me./g. ester. Theresidue amounted to 350 g. and contained 2.50 me./g.-ester, indi catingthat some esterification took place during distillation. The distillateobtained was reacted with 271 g. pentaerythritol under a nitrogenblanket, in the presence of 50 cc. of xylene acting as an azeotropicagent. The temperature of the reactants was raised to 215 C. overaperiod of about 5 hours, while the acid content dropped to 0.08 me./g. Atotal of cc. water of esterification was collected, which correspondedapproximately to the theoretical amount calculated. After removal ofxylene, the reactants were cooled to C., and then a vacuum of 2.5 mm. Hgwas applied and non-acids (i.e. unreacted and partially reactedoxidation products) were strippedoff for a period of time until a vaportemperature of C. was reached. At this stage 431 g. of non-acids werecollected, leaving a total of 1189 g. of the fatty acid-polyol ester.After cooling, the polyol ester portion was reacted with 1,200 ml. ofmethanol in the presence of 4 g. NaOCI-I at reflux temperatures forabout 30 minutes,

whereupon the excess methanol was stripped-01f and the precipitatedpentaerythritol was filtered off. Thus, 1080 g. methyl ester with 246 g.pentaerythritol were obtained. Recovery of the pentaerythritol amountedto about 91% and was found to be of excellent quality ready for recycle.The methyl ester had a mean molecular weight of 179.4 and an estercontent of 95.2%, as determined by gas chromatography and 94.8% asdetermined by Ester Value. Upon distilling the methyl ester under 2.5mm. Hg vacuum, 90% went over a maximum vapor temperature of 145 C. Theobtained distillate was colorless and contained only 1.1% impurities, asdetermined by gas chromatography.

The residue left upon distillation of the crude acids as described atthe beginning of this example was then submitted to a high pressure-hightemperature saponification and steam distillation at about 300 C. and2,000 p.s.i. In this manner 238 g. crude acids and 68 g. recycle oil wasobtained from 350 g. residue. Upon high-vacuum distillation of the crudeacids, 139 g. of a colorless material was collected, which uponesterification with methanol, was analyzed by gas chromatography tocontain only 3.1% impurities. The material balance of the wholeoperation was consequently as follows:

Total products obtained: 1213 g. or 63% yield Recycle material: 499 g.or 26% yield EXAMPLE IV A reaction was carried out as described inExample III, except that the crude fatty acids were reacted directlywith pentaerythritol, without previous distillation. The recoveredpentaerythritol amounted to 79% of polyol charged, the loss beingattributed to interference by polyfunctional materials in the crudeproducts.

EXAMPLE V A reaction was carried out in accordance with the procedureoutlined in Example III, except that glycerol was substituted forpentaerythritol. Upon interesterification with an excess amount ofmethanol, the glycerol layer dropped out almost quantitatively as adistinct layer, and the obtained methyl ester had an impurity content of3.8%, as determined by gas chromatography. The recovered glycerol wasfound to necessitate some purification before being in condition forreuse, however, under certain conditions this recovered glycerol layercould be utilized without such purification.

17 EXAMPLE v1 Example HI was mixed with a solution of 90 g. NaOH in 1500cc. H O. The obtained soap solution was then charged into an autoclaveconstructed of 316 stainless steel of one gallon capacity and equippedwith agitator, internal cooling and an automatic temperature controlassembly. A narrow tube connected to the top of the pressure vessel witha long condenser. The system was closed and air purged therefrom andpressurized hydrogen added. Heat was applied until a temperature ofabout 330 C. was attained while pressure rose to 1980 p.s.i. After a onehalf hour digestion period, steam was allowed to escape through thecondenser until a total of 1000 ml. water was collected, together with88 g. of steam-entrained oil. The concentrated soap solution (i.e. about50% of strength) was cooled, discharged and acidified with H SO to a pH3.5 and 397 g. crude fatty acids was obtained by decantation. Such crudefatty acids were esterified with methanol and the obtained methyl esterwas distilled under 4 mm. vacuum until the vapor temperature reached 183C. In this manner 232 g. of distillate and 139 g. of residue wasobtained. The distillate had a mean molecular weight of 190.5 andcontained 3.1% of impurities as determined by gas chromatography.

EXAMPLE VII 675 g. of the soap solution obtained in the reactiondescribed in Example VI was dispersed in 1.5 liters of methanol andacidified with 160 g. of concentrated H 80 by slow addition underrefluxed conditions. A copius precipitate of Na SO fell out and wasremoved by filtration. Upon drying, the precipitate weighed 200 g.Methanol was then stripped-off until a total weight of 1300 g. remainedwithin the reaction apparatus. This was cooled to room temperature andan additional amount of 166 g. H 80 was introduced, which caused atwo-layer system to form. The layers were separated, by decantation, andthe upper layer weighed 419 g. and was analyzed to consist essentiallyof pure methyl esters while the bottom layer, upon dilution with water,yielded 100 g. of oil consisting of a methyl ester with a high contentof impurities (23.2% The methyl ester recovered from the upper layer wasanalyzed to have a mean molecular weight of 186 and to contain 8.1%impurities, as determined by gas chromatography.

EXAMPLE VIII 80 g. of a methyl ester prepared from the crude syntheticfatty acids obtained by the reaction described in Example I was analyzedby gas chromatography to contain 20.2% impurities and was well agitated,at room temperatures, with a solvent mixture of 160 g. methanol and 75g. sulfuric acid solution (i.e. having a concentration less than 94%).After standing a few minutes, the system separated into two distinctlayers, which were separated one from the other by decantation. The toplayer weighed 54 g. and was analyzed to consist essentially of methylesters having a mean molecular Weight of 192 and containing 11.4%impurities. The bottom layer was diluted with 500 g. H and 24 g. of oilseparated out which was analyzed as consisting essentially of a methylester having a lower mean molecular weight (181.4) and containing 31.4%impurities.

The efficiency coeificient, defined as the ratio between the amount ofimpurities removed and the amount of total material removed, was =0.38for this example.

EXAMPLE IX The procedure described in Example VIII was repeated, exceptthat the solvent mixture was 153 g. of a 1:1 mixture of methanol andconcentrated H SO' This time the top layer weight 66.5 g. and the methylester contained 10.6% impurities and the efiiciency coefiicient was0.675.

The top layer was extracted a second time with the same amount ofsolvent mixture. The recovered methyl ester weighed 58.5 g. andcontained 8.1% of impurities while the efliciency coefficient was 0.29.

EXAMPLE X g. of a synthetic methyl ester containing 13.4% impurities asdetermined by gas chromatography was shaken vigorously with g. of a 1:1mixture of acetone and concentrated H 50 After allowing the system tostand a few minutes, the layers were separated and the top layer washedwith water. The methyl ester obtained weighed 59.3 g. and contained 6.5%of impurities. The efliciency coeflicient was 0.334.

EXAMPLE XI A C -C fraction of a synthetic methyl ester obtained by theprocess described in Example HI was reacted with a calculated amount ofdiethanolamine in the presence of NaOCH until the amidification reactionwas practically complete. The resultant amide had a color factor of 5 onthe Gardner scale. 60 g. of this amide was diluted with 500 ml.methanol, in order to lower the viscosity thereof, and the solution wasplaced in a stainless steel pressure vessel together with 12 g. of a 50%nickel on kieselgur catalyst. Air was purged from the vessel withhydrogen at a pressure of about 500 psi. and heat was added until atemperature of about 80 C. was obtained. Thereafter, the vessel wasclosed and these conditions were maintained for about 4 hours. Thematerial was then cooled, depressurized, discharged and the catalystremoved by filtration. After stripping the methanol, the resultant amidehad a color factor of 2 on the Gardner scale.

EXAMPLE XII 1200 g. of a mixture of linear tetradecane, pentadecane, andhexadecane was placed in a 2-liter flask provided with a stirrer, athermometer, a gas inlet and a scrubbing assembly. 2 g. of manganeselaurate was added and the contents in the flask heated to a temperatureof C. while air was sparged through this mixture at a rate of I lit./min. Two drops of di-tertiary butyl peroxide was then introduced and theoxidation reaction started immediately. The above described conditionswere maintained for about 16 hours, at which time the free acid contenthad reached the value corresponding to 38% by weight of the reactants.The air flow was then stopped, and the contents of the flask cooled tobelow 80 C. and the particulate catalyst removed by filtration.Thereafter 75 g. of pentaerythritol was introduced together with 50 g.of xylene and the resulant solution was heated gradually up to 220 C.,with a continual withdrawal of water of esterification azeotropicallyunder a blanket of nitrogen. When the free acid content had dropped to0.08 me/g., vacuum was gradually applied while the temperature wasallowed to drop to 180 C. The vacuum distillation was continued untilthe vapor temperature reached 210 C. under 3 mm. Hg vacuum. Thereactants were then cooled to 60 C. and 500 ml. of dry methanol wasintroduced therein together with 5 g. of a 25% solution of NaOCH Almostimmediately a voluminous precipitate of pentaerythritol was formed. Thereactants were further digested at 70 C. for a period of about 30minutes and then cooled to room temperature. The precipitate (i.e.pentaerythritol) was then filtered off and after drying in an oven theprecipitate weighed 54.5 g., for a recovery of about 78% polyol. Therecovered polyol was pure enough for reuse without additionalpurification. 'Ihe methanolic solution in the filtrate contained methylesters which upon fractionation yielded a product of high purity.

It is therefore clear that the present invention provides methods ofpurification of carboxylic compounds obtained 19 by a continuousoxidation process wherein the free fatty acid content is maintainedbelow 3% in the reaction zone, as well as methods of obtaining pureesters of synthetic fatty acids obtained by a batch process wherein theamount of free fatty acids is allowed to rise as high as 40% in thereaction zone.

Throughout the instant disclosure and the claims thereof, the term fattyacid (low molecular) ester or fatty acid '(C -C ester is frequentlyused. The term as utilized herein will be understood to mean fatty acidesters of low molecular weight alcohols or fatty acid esters of C -Calcohols.

Various other modifications and changes, other than those alreadydiscussed, can of course be effected without departing from the spiritand scope of the novel concepts of the instant invention.

I claim as my invention:

1. A process of obtaining straight-chain monobasic fatty acid esters ofrelatively low molecular weight aliphatic alcohols from C through Cessentially linear hydrocarbons of an average molecular weightcorresponding to hydrocarbons having 2 to 6 more carbon atoms than thefatty acid precursors of a desired range of fatty acid esters of saidalcohols, consisting essentially of (A) substantially uniformlydispersing an organic peroxide catalyst having a half life of to 100minutes at a temperature range of 120 to 180 C. with said linearhydrocarbon to obtain a mixture thereof;

(B) heating said mixture to a temperature of not more than about 160 C.;

(C) substantially simultaneously contacting said mixture with an oxygencontaining gas to effect an oxidation thereof not exceeding about 40% offree fatty acids by weight content of said mixture;

(D) adding and intermixing an alkaline solution with at least a portionof said mixture to effect a phase separation of the resultant mixture;

(E) removing an upper phase of said resultant mixture and recycling saidupper phase to step (B);

(F) acidifying the remaining lower phase of said resultant mixture withan equivalent amount of a mineral acid so as to obtain a phaseseparation whereby an upper phase thereof contains fatty acid precursorsof the desired range of fatty acid esters;

(G) adding a relatively high boiling polyol selected from the groupconsisting of pentaerythritol, glycerol and glycols to said upper phaseof step (F) and substantially simultaneously heating the ensuing mixtureto form a reaction mixture consisting of fatty acid polyol esters andimpurities, said fatty acid polyols having a boiling range substantiallyhigher than the boiling range of said impurities;

(H) continuously removing water of esterification from said ensuingmixture and controlling the temperature thereof;

(I) cooling said reaction mixture to substantially below the boilingrange of said impurities;

(I) applying vacuum and controlled heat to the cooled reaction mixtureto remove said impurities therefrom;

(K) adding at least five equivalents of said low molecular weightaliphatic alcohol in the presence of an interesterification catalyst toattain a mixture of said fatty acid polyols and said low molecularweight alcohol;

(L) refluxing the attained mixture under ester-forming conditions for aperiod of time ranging from about 15 to 60 minutes so as to form a layerof fatty acid esters of said low molecular weight aliphatic alcohol anda layer of said high boiling polyol; and

(M) separating said layers to recover said high boiling polyol and thedesired range of fatty acid esters of said low molecular weightaliphatic alcohol.

2. A batch process of obtaining straight-chain monobasic fatty acidesters Qf I l'sliively low molecular weight 20 aliphatic alcohols from Cthrough C essentially linear hydrocarbons of an average molecular weightcorresponding to hydrocarbons having 2 to 6 more carbon atoms than thefatty acid precursors of a desired range of fatty acid esters of saidalcohols, consisting essentially of (A) substantially uniformlydispersing an organic peroxide catalyst having a half life of 5 tominutes at a temperature range of to 180 C. with said linear hydrocarbonto obtain a mixture thereof;

(B) heating said mixture to a temperature of not more than about C.;

(C) substantially simultaneously contacting said mixture with an oxygencontaining gas to effect an oxidation thereof equal to about 10% to 40%of free fatty acids by weight content of said mixture;

(D) adding and intermixing an alkaline solution with at least a portionof said mixture to etfect a phase separation of the resultant mixture;

(E) removing an upper phase of said resultant mixture and recycling saidupper phase to step (B);

(F) acidifying the remaining lower phase of said resultant mixture withan equivalent amount of a mineral acid so as to obtain a phaseseparation whereby an upper phase thereof contains fatty acid precursorsof the desired range of fatty acid esters;

(G) adding a relatively high boiling polyol selected from the groupconsisting of pentaerythritol, glycerol and glycols to said upper phaseof step (F) and substantially simultaneously heating the ensuing mixtureto form a reaction mixture consisting of fatty acid polyol esters andimpurities, said fatty acid polyols having a boiling range substantiallyhigher than the boiling range of said impurities;

(H) continuously removing water of esterification from said ensuingmixture and controlling the temperature thereof;

(I) cooling said reaction mixture to substantially be low the boilingrange of said impurities;

(J) applying vacuum and controlled heat to the cooled reaction mixtureto remove said impurities therefrom;

(K) adding at least five equivalents of said low molecular weightaliphatic alcohol in the presence of an interesterification catalyst toattain a mixture of said fatty acid polyols and said low molecularweight alcohol;

(L) refluxing the attained mixture under ester-forming conditions for aperiod of time ranging from about 15 to 60 minutes so as to form a layerof fatty acid esters of said low molecular weight aliphatic alcohol anda layer of said high boiling polyol; and

(M) separating said layers to recover said high boiling polyol and thedesired range of fatty acid esters of said low molecular weightaliphatic alcohol.

3. A continuous process of obtaining straight-chain monobasic fatty acidesters of relatively low molecular weight aliphatic alcohols from Cthrough C essentially linear hydrocarbons of an average molecular weightcorresponding to hydrocarbons having 2 to 6 more carbon atoms than thefatty acid precursors of a desired range of fatty acid esters of saidalcohols, consisting essentially of (A) substantially uniformlydispersing an organic peroxide catalyst having a half life of 5 to 100minutes at a temperature range of 120 to C. with said linear hydrocarbonto obtain a mixture thereof;

(B) heating said mixture to a temperature of not more than about 160 C.;

(C) substantially simultaneously contacting said mixture with an oxygencontaining gas to effect an oxidation thereof not exceeding about 3% offree fatty acids by weight content of said mixture;

(D) adding and interm'uring an alkaline solution with at least a portionof said mixture to effect a phase separation of the resultant mixture;

(E) removing an upper phase of said resultant mixture and recycling saidupper phase to step (B);

(F) acidifying the remaining lower phase of said resultant mixture withan equivalent amount of a mineral acid so as to obtain a phaseseparation whereby an upper phase thereof contains fatty acid precursorsof the desired range of fatty acid esters;

(G) adding a relatively high boiling polyol selected from the groupconsisting of pentaerythritol, glycerol and glycols to said upper phaseof step (F) and substantially simultaneously heating the ensuing mixtureto form a reaction mixture consisting of fatty acid polyol esters andimpurities, said fatty acid polyols having a boiling range substantiallyhigher than the boiling range of said impurities;

(H) continuously removing water of esterification from said ensuingmixture and controlling the temperature thereof;

(I) cooling said reaction mixture to substantially below the boilingrange of said impurities;

(J applying vacuum and controlled heat to the cooled reaction mixture toremove said impurities therefrom;

(K) adding at least five equivalents of said low molecular weightaliphatic alcohol in the presence of an interesterification catalyst toattain a mixture of said fatty acid polyols and said low molecularweight alcohol;

(L) refluxing the attained mixture under ester-forming conditions for aperiod of time ranging from about 15 to 60 minutes so as to form a layerof fatty acid esters of said low molecular weight aliphatic alcohol anda layer of said high boiling polyol; and

(M) separating said layers to recover said high boiling polyol and thedesired range of fatty acid esters of said low molecular weightaliphatic alcohol.

4. A process as defined in claim 1 wherein the upper organic phaseobtained at step (F) is subjected to reduced pressures and elevatedtemperatures for flash distillation of a majority of any mono-functionalcompounds present therein prior to step (G).

5. A process as defined in claim 1 wherein step (G) is conducted under ablanket of inert gas and step (H) consists of adding an azeotropic agentto the reaction mixture while maintaining the temperature thereof belowabout 300 C.

6. A process as defined in claim 1 wherein step (H) is terminated when afree acid content in the range of about 0.005 to 0.5 milliequivalentsper gram of reaction mixture is attained.

7. A process as defined in claim 1 wherein step (I) comprises applying avacuum in the range of 2 to 10 mm. Hg to the reaction mixture whilesubstantially simultaneously maintaining the temperature thereof aboutbelow the boiling range of the fatty acid polyol esters.

8. A process as defined in claim 1 wherein the polyol ispentaerythritol.

9. A process as defined in claim 1 wherein the aliphatic alcohol ismethanol.

10. A process of obtaining straight-chain monobasic fatty acid 0, to Cesters from crude soap solutions of fatty acid products obtained fromoxidation of C through essentially linear hydrocarbons of an averagemolecular weight corresponding to hydrocarbons having 2 to 6 more carbonatoms than the fatty acid precursors of a desired range of fatty acid Cto C esters, consisting essentially of feeding a crude soap solution ofabout 10% to 40% strength into a pressure vessel, mixing the crude soapsolution with a hydrogenation catalyst, purging air from said vessel,adding hydrogen gas to said vessel to obtain a pressure in the range ofabout 350 to 650 p.s.i. within said vessel while substantiallysimultaneously raising the temperature within said vessel to about 190C. to 300 C., maintaining said temperature and resultant pressureconditions within said vessel for a period of time ranging from about 10to 60 minutes, slowly releasing the pressure and allowing volatiles toescape until a soap solution of about 50% to strength is obtained,cooling the relatively concentrated soap solution to below about 70 C.,removing the hydrogenation catalyst, adding 1 to 3 volume equivalents ofa C to C aliphatic alcohol, reacting the resultant solution with astoichiometric amount of sulfuric acid to form a sodium sulfate salt,allowing the sodium sulfate salt to precipitate and removing said saltfrom solution while maintaining the temperature thereof in the range ofabout 10 C. to 50 0., adding a relatively small amount of sulfuric acidto the solution to effect a phase separation therein, and decanting thetop layer thereof containing the desired range of pure fatty acid estersof said C to C alcohol.

11. A method as defined in claim 10 wherein the mixing the organic layerwith the sulfuric acid to form a two-layer system and separating the twoformed layers of such system is repeated a plurality of times.

12 A method as defined in claim 11 wherein the repetition of mixing andseparating is carried out by a countercurrent flow process.

13. A method as defined in claim 10 wherein the C to C alcohol ismethanol.

14. A method of purifying esters of synthetic fatty acids obtained byoxidation of C through C essentially linear hydrocarbons of an averagemolecular weight corresponding to hydrocarbons having 2 to 6 more carbonatoms than the fatty acid precursors of the desired range of fatty acidesters, consisting essentially of; mixing said esters with a relativelysmall amount of sulfuric acid of a strength ranging from about 60% to94% so as to form a two-layer system, separating said layers whereby thelower layer contains sulfuric acid and a majority of any impurities insaid esters and the upper layer contains a majority of said esters andan amount of impurities substantially smaller than the original amountof impurities.

15. A method as described in claim 14 wherein the process of admixing arelatively small amount of sulfuric acid and subsequent separating ofthe formed layers is repeated a plurality of times.

16. A method as described in claim 15 wherein the repetition is carriedout by a counter-current flow process.

References Cited UNITED STATES PATENTS 1,278,198 9/1918 Oberfell et al.260-499 2,383,633 8/1945 Trent 260-4109 2,783,270 2/1957 Poly et a1.260485 2,987,536 6/ 1961 Skees et al 260-451 FOREIGN PATENTS 6,067,7118/ 1948 Great Britain.

LEWIS GOTTS, Primary Examiner D. G. RIVERS, Assistant Examiner US. Cl.X.R.

